I'm just going through the same thing with my TDS784D, where the channel 3 BNC connector is marginal. I have a "spare", a probably-dead TDS684A, which has the same attenuator assembly.
At first glance the outer flange ring is very small and the 12 mm bnc will not fit inside, the 131536100 is 10 mm. For the length i can pry the metal flange so 1-2 mm more or less is not an issue. The thread seem standard. i found some 131536100 image and it look like the one in the casing. Well i think this put and end on this project for now.
I got the set screw out - it was very hard to get moving, but once it snapped it was fine. Someone on the TekScopes forum suggested that heating the set screw with e.g. a soldering iron will soften the locktite.
I removed a BNC connector from my spare assembly to see what they look like. It was a bear to get out, and I totally wrecked the connector in the process. Heat might have been a good idea, but the attenuator assembly is fine. After cleaning out the residual locktite off connector and assembly with IPA, the threads feel just fine.
Now, the measurements in the data sheet you uploaded are all bang-on to the connector I pulled.
It looks like a pretty regular BNC, except it's long - 14.5mm from the back of the flange to the front of the connector. More usual is to see ~12mm; this from staring at mechanical drawings on e.g. DigiKey. The flange is also smaller than typical, it's 10.8mm instead of the more usual 12-12.5mm.
I found that QService had a couple of suitable BNCs on stock, but after ordering them I noticed that those are used - so I don't know what sort of state they'll be in.
Note that there's a code stamped on the front of the connector, which on mine reads 28JR472–1. This matches the part number in the TDS520B service manual, and this part number turns up a few suppliers (hoarders?) on the interwebs who claim to have them in stock.
See e.g. <http://www.1sourcecomponents.com/availability/Berg--28JR472-1.htm>, who claim to have 2198 in stock.
Has anone here done business with this ilk of supplier?
Alternatively I noticed that the TDS520B SM has a few mentions of the same part number. I suspect the connectors on the back of the scope are the same kind, and maybe it's viable to swap those to the front. I figure it must be easier to replace the back ones with easier-to-find parts.
Siggi